Retroviral entry into cells depends on envelope glycoproteins, whereby receptor binding to the SU subunit triggers membrane fusion by the transmembrane (TM) subunit. The human T cell leukemia virus type 1 (HTLV- 1) is a retrovirus with a wide geographic distribution, associated with adult T cell leukemia and tropical spastic paraparesis/HTLV- I -associated myelopathy. We have crystallized the ectodomain of gp2 1, the TM from (HTLV- 1), as a maltose-binding protein (MBP) chimera. The rhombohedral crystals have the symmetry of the space group R3 with cell dimensions a = b = 102.9 A, c = 118.0 A and angles alpha = beta = 90, gamma = 120 degrees, and diffract to a resolution of 2.5 A in house (exposure times -I h/I degree oscillation). The effective resolution is lower due to the anisotropy of the diffraction pattern. No diffraction data have yet been collected with synchrotron radiation using the optimized crystals. A high-resolution monochromatic cryo data set is required for drug design efforts using the structure of gp2l. Data was collected on BioCARS Station 14-BM-C.